A historic low in the number of Monarch butterflies overwintering in a wooded sanctuary in central Mexico is prompting experts to warn that the insects’s famed annual migration from Canada and the United States could fade away soon.

“The news is bad, really bad,” said Omar Vidal, the World Wildlife Fund’s Mexico director. “The Monarch migration, the symbol of co-operation between the three countries, is at serious risk of disappearing.”

The WWF has been monitoring the size of the colonies of butterflies spending the winter in a particular patch of fir forest that straddles the central states of Michoacán and the State of Mexico since 1993.

The largest area occupied by the butterflies was recorded in 1997 and reached 44.5 acres (18 hectares). This season, the area fell to 1.65 acres (0.67 hectares).

“There have always been ups and downs in the populations, but over the last 10 years we have seen a clear downward tendency,” Vidal said.

With illegal logging in the sanctuary now largely under control, experts are putting most of the blame for the current crisis on the disappearance of the Monarchs’s milkweed breeding grounds in the US and Canada.

Karen Obenhauser, a conservation biologist from the University of Minnesota who has been studying Monarch migration for 30 years, says this is primarily because of the explosion in the use of herbicide-resistant genetically modified corn and soybean. The consequent rise in the use of herbicides is killing other plants present in the fields, such as milkweed.

“There is an urgent need to replace the habitat that has been lost,” she said.

Scientists say uncharacteristically extreme weather, associated with climate change, poses another serious threat to butterfly populations, although the species itself is not in danger of extinction.

While in their Mexican woodland sanctuary, the Monarchs protect themselves from cold nights by huddling together in huge clumps hanging from the boughs of the trees that transform into clouds of orange and black when the sun comes out.

Scientists have yet to understand how it is that the butterflies return to the same Mexican forest every winter, particularly given that there are several generations involved in the journey that can cover 4,000 kilometres (2,486 miles).

There are also other overwintering sites in California, although these are much smaller, as is the distance travelled by the butterflies that gather there.